CHAPTER 22 Renal Disorders 535
electrolytes leave the blood and pass into the proxi- mal tubule. At this point, the glomerular filtrate is very dilute and contains a high amount of electrolytes, glucose, and metabolic waste products. At the proximal tubule, approximately 60% of water is reabsorbed back into the bloodstream. As the tubule fluid travels through the various parts of the nephron, water and electrolytes such as sodium and potassium move to and from tubule fluid and blood. Within the next section of the nephron, called the loop of Henle, urea , a composite of nitrogenous waste that needs to be excreted, is secreted into the tubule fluid. At this juncture within the nephron, the tubule fluid, which contains urea, starts to resemble the finished prod- uct: urine. Overall, the loop of Henle reabsorbs about 25% of filtered electrolytes, such as sodium, chlorine, potassium, calcium, and bicarbonate, and 15% of the filtered water. At the distal tubule, aldosterone acts to reabsorb more sodium and water into the bloodstream and secrete potassium into the tubule fluid. Here again, tubule fluid is further concentrated and the body saves water. Finally, at the collecting duct, under the influ- ence of antidiuretic hormone, the last amount of water needed by the body is reabsorbed from the tubule fluid back into the bloodstream. At this last stage, the highly concentrated tubule fluid is urine (see Fig. 22-2). Acid–Base Balance Normal body function is dependent on acid–base bal- ance, and the kidneys play a major role in this through the regulation of bicarbonate and hydrogen reab- sorption or secretion. Acids are produced during nor- mal metabolic processes, requiring the physiological response of buffering to maintain the physiological pH of 7.35 to 7.45. The kidneys’ role in maintaining acid–base balance involves excretion or conservation of hydrogen ions [H + ] and bicarbonate ions [HCO 3 – ]. Waste Elimination During the cell’s metabolic activity, waste products are accumulated. These waste products include such substances as urea, uric acid, creatinine, and drug metabolites. If not excreted in the urine, waste prod- ucts become toxic to body tissues, particularly break- down products of drugs. A reduction in renal function can prolong the effect of some medications, which can lead to adverse effects or toxicity. Secretory Functions The kidney has several unique secretory functions that are triggered by certain conditions in the body. Hypoxia and low blood volume are two such condi- tions. Hypoxia stimulates erythropoietin secretion by the kidney. Low blood volume stimulates renin secre- tion by the kidney.
Bowman's capsule
Distal tubule
Proximal tubule
Glomerulus
Loop of Henle
Collecting duct
Control of Blood Pressure The major mechanism whereby the kidneys influ- ence systemic blood pressure and blood volume is the renin–angiotensin–aldosterone system (RAAS) . The RAAS contributes to sodium and water reabsorp- tion into the bloodstream and potassium excretion at the renal tubules. A specialized region of the neph- ron called the juxtaglomerular apparatus is sensi- tive to sodium. This is the specific region around the glomerulus in each nephron. These cells sense low sodium and, in response, secrete renin. Other triggers for renin secretion include decreased renal perfusion and increased sympathetic nervous system activity. The net effects of the RAAS activity are sodium and water reabsorption, potassium excretion, and arterial vasoconstriction. FIGURE 22-2. Basic functions of the nephron. The nephron’s basic goal is to yield a concentrated urine that contains waste products. The blood and tubule fluid undergo a great deal of exchange before the tubule fluid becomes urine. The glomer- ulus is a tuft of capillaries from which blood is filtered at the Bowman’s capsule. The glomerulus allows substances such as water, sodium, bicarbonate, acids, and urea out of the blood. However, the glomerulus does not allow large proteins such as albumin out of the blood. At the proximal tubule, a large amount of water, sodium, and potassium are reabsorbed into the bloodstream. At the descending loop of Henle, a high amount of sodium is reabsorbed, and urea is secreted from the blood into the tubule. Aldosterone, a hormone secreted by the adrenal gland, increases sodium and water reabsorption. In the distal tubule, sodium and water are reabsorbed from the tubule fluid into the bloodstream and urine is formed. If the body needs more water, antidiuretic hormone (ADH) from the posterior pituitary works at the collecting duct to increase water reabsorption into the bloodstream for a more concentrated urine.
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